An apparatus for producing films or layers on substrates or wafers by conveyorized atmospheric pressure chemical vapor deposition (APCVD) is described in U.S. Pat. No. 4,834,020 and owned by the assignee of the present invention. This patent is expressly incorporated by reference herein.
In general, an APCVD apparatus includes a conveyor belt which transports a wafer or substrate into one or more coating chambers. The coating chambers include means for creating and maintaining a chemical vapor atmosphere at the wafer or substrate surface such that a reaction of the chemical vapors with the wafer or substrate can transpire and thereby produce a deposited film on the wafer or substrate.
An important element in creating and maintaining the chemical vapor atmosphere at the wafer surface is the exhaust system. The gases must be introduced and exhausted at a uniform rate across the wafer or substrate in order to provide the proper residence time for the reacting chemicals over the surface.
A variety of problems result when the exhaust system does not function properly. When the chamber is exhausted too quickly it results in a loss of deposition. If the chamber is exhausted too slowly, the gas flows are undefined, resulting in particle generation. If the exhaust rate fluctuates with time, substrate uniformity and particle control is compromised.
Thus, to avoid these problems and otherwise insure proper functioning of the APCVD apparatus, one must precisely control the exhaust flow rate. A traditional solution is to install a flowmeter in each exhaust or vent line of the exhaust system. This solution, however, is of limited usefulness since the byproducts of the reaction from the chemical vapor are generally abrasive, corrosive, and adherent. Consequently, the vent line accumulates debris and renders the flow meter inoperative.
Another solution is to put a restriction in each vent line and thereby create an orifice restriction area. Flow through an orifice is proportional to the orifice's area times the square root of the pressure drop. Therefore, if the orifice is small, the pressure differential is large and the flow can be easily and accurately controlled by the pressure. Unfortunately, as debris accumulates around the orifice restriction area, the orifice changes size and flow control is lost.
To prevent this debris accumulation which hampers the proper functioning of the exhaust system, self-cleaning orifices have been devised. One such device utilizes a wire to regularly wipe the orifice. Another device utilizes opposing rollers and wipers to create and maintain a constant orifice restriction area.
The problems associated with these prior art solutions are readily recognizable. Utilization of the wire yields limited results since the wire is capable of little more than rudimentary swipes at the orifice. In addition, the wire only cleans the orifice itself and does not clear the debris which gathers over the entire orifice restriction area or on the wire itself. This debris obstructs normal gas flow.
The roller solution presents its own class of problems. First, the design relies upon concentric parts machined to exacting tolerances. Manufacturing such devices is difficult. Once manufactured and installed, it is difficult to maintain a constant orifice. Moreover, the design is difficult to seal; as a result, toxic chemicals can leak from the tube connections and orifice roller shafts. Aside from the health problems associated with the toxic chemicals leaking from an exhaust system, those chemicals tend to adhere to the rollers, notwithstanding the wipers, causing the rollers to jam, or even stop turning.
Under either approach, one is forced to stop the apparatus periodically to provide a thorough cleaning of the orifice restriction area in order to insure proper functioning of the exhaust system.